Deploying optical network elements (ONEs) to form an optical network is a difficult and expensive proposition: network providers need to correctly anticipate customer demand while building reliable networks as inexpensively as possible. In addition, network providers must also anticipate future technological developments, such as increased data rates, to simplify network upgrades. In part, network providers attempt to minimize cost and reduce network complexity by deploying ONEs, such as optical amplifiers and optical regenerators, in a way that minimizes the required power while ensuring signal fidelity.
In digital communication schemes, such as those employed in optical networks, signal fidelity may be characterized by a bit error rate (BER). Simply put, the BER is how frequently a receiver detects a bit incorrectly, that is, how often the receiver mistakes a representation of a logical ‘1’ for a representation of a logical ‘0’ or vice versa. Lower BERs are better; ideal (i.e., noise-free) receivers operate with BERs of zero (0), but shot noise and thermal noise at real receivers cause bit detection errors, raising BERs to measurable levels.
Currently, the target BER for optical networks is on the order of 10−12. To meet the target BER, network providers must guarantee a minimum optical signal-to-noise ratio (OSNR) at the receiver. The OSNR is usually defined as the ratio of the optical signal power Ps to the optical noise power Pn in a given channel bandwidth,
                              O          ⁢                                          ⁢          S          ⁢                                          ⁢          N          ⁢                                          ⁢          R                =                  10          ·                                                    log                10                            ⁡                              (                                                      P                    s                                                        P                    n                                                  )                                      .                                              Equation        ⁢                                  ⁢        1            For digital signals, the detected power switches between a high level and a low level at a given data, or bit rate. In optical networks, the high and low levels can be defined in terms of a number of photons: for example, a 5 mW, 40 GHz optical signal in the Wavelength Division Multiplexing (WDM) C band may have a corresponding high level of about 106 photons and a low level of 0 photons. In a shot-noise limited receiver, a signal of 106 photons has an OSNR of 30 dB.
Because bits can be defined in terms of photons, the bit rate can be defined in terms of photons per second. As the bit rate increases, the number of photons per bit decreases given a constant optical power (i.e., spreading a constant number of photons per second over a larger number of bits per second reduces the photons per bit). The increased bit rate also leads to a decreased OSNR—the bandwidth increases, but the signal power remains constant, whereas the receiver noise power increases given a relatively constant noise power spectral density. Eventually, increasing the bit rate depresses the OSNR too far, pushing the BER above acceptable levels. In optical networks that use direct detection (i.e., networks that use on/off keying), the BER is related to the OSNR according to the relation
                                          B            ⁢                                                  ⁢            E            ⁢                                                  ⁢            R                    ∝                                    1              2                        ·                                          log                10                            ⁡                              (                                  O                  ⁢                                                                          ⁢                  S                  ⁢                                                                          ⁢                  N                  ⁢                                                                          ⁢                  R                                )                                                    ,                            Relation        ⁢                                  ⁢        2            where the OSNR is in linear units. As shown in Relation 2, maintaining a minimum BER requires maintaining a minimum OSNR. This, in turn, means that any increase in the bit rate should be offset by a corresponding increase in the OSNR to keep the BER at acceptable levels.
As light propagates through a network, however, it is absorbed and scattered, reducing the signal power and the OSNR. In addition, signals propagating through optical fiber suffer from loss due to four-wave mixing, chromatic dispersion, and polarization mode dispersion, further reducing the OSNR. As stated above, reductions in OSNR hamper the network's ability to support higher bit rates.
In long-haul and metro optical networks, optical amplification may boost the signal power to reliably detectable levels. Amplifiers add noise to the signal, however, despite increasing the signal strength. Even ideal amplifiers double the amount of noise present, which corresponds to a reduction of the OSNR by 3 dB.
Optical regeneration restores degraded signals to detectable status using optical-to-electrical-to-optical conversion. The degraded optical signals are converted to electrical signals, which can be processed in the electrical domain before being converted back to the optical domain. The resulting optical signals may have high enough OSNRs to be detectable throughout the network. Unfortunately, the transponders required to regenerate optical signals are complex and costly. Worse, their complexity and cost increase with the data rate and the number of channels, making regeneration an unattractive option for maintaining OSNR throughout an optical network.